The distribution of moisture across the globe is governed by massive, predictable currents of air circulating high above the surface. This atmospheric movement provides the geographical explanation for the location of the world’s largest and most persistent arid regions. Understanding how air masses move and interact with heat energy reveals the mechanism that draws water from one part of the world and leaves vast swaths of land perpetually dry. These global circulation patterns are the direct cause of the great subtropical deserts that ring the planet.
How Air Movement Controls Moisture
The fundamental link between air movement and precipitation connects temperature and the air’s capacity to hold water vapor. When air near the ground is warmed, it becomes less dense and begins to ascend through convection. This rising air encounters lower atmospheric pressure as it gains altitude, causing it to expand.
As the air expands, the energy driving its molecules spreads over a larger volume, resulting in a drop in temperature, known as adiabatic cooling. Cooler air has a reduced capacity to hold water vapor, causing the relative humidity to climb. Once the air cools past its saturation point, the water vapor condenses into liquid droplets, forming clouds and leading to precipitation. Areas of sustained rising air are characterized by heavy rainfall.
Conversely, when air descends toward the surface, it encounters increasing atmospheric pressure. This compression causes the air to warm rapidly, known as adiabatic heating. Since the total moisture remains the same, this temperature increase drastically lowers the relative humidity. The warming air becomes stable, suppressing cloud formation and actively drawing moisture from the land below. This downward motion creates an environment where precipitation is nearly impossible, making the land extremely dry.
The Global Engine: Understanding Hadley Cells
The intense solar radiation near the equator drives the largest atmospheric circulation system on Earth, known as the Hadley Cell. This system begins when the Sun heats the surface air most directly, causing it to become buoyant and rise through convection. This zone of intense uplift, where air masses from both hemispheres converge, is called the Intertropical Convergence Zone (ITCZ).
The warm, moist air rising in the ITCZ ascends to high altitudes before spreading out and flowing poleward. As this air rises and cools adiabatically, it sheds moisture, resulting in the heavy rainfall that nourishes tropical rainforests. By the time this air mass begins its horizontal journey away from the equator, it is largely depleted of its water vapor content.
The air continues its high-altitude flow toward the poles, losing heat through radiation. Around 30 degrees latitude in both hemispheres, the air cools sufficiently to become dense enough to begin sinking back toward the surface. This poleward movement and subsequent sinking complete the circulation loop that defines the Hadley Cell.
Sinking Air Creates Desert Zones
The downward motion of the poleward-moving air mass is the direct cause of the world’s subtropical deserts. As this dry, cool air sinks around 20 to 30 degrees latitude, it creates an area of persistent high pressure known as the Subtropical High Pressure Belt. This region is characterized by calm, stable atmospheric conditions that inhibit upward movement and cloud formation.
The air descending from the upper atmosphere undergoes compression and adiabatic heating as it approaches the surface. This warming effect superheats the already moisture-depleted air, increasing its capacity to hold water. Any residual moisture in the lower atmosphere is quickly absorbed by the sinking, warming air, preventing condensation and suppressing precipitation.
The resulting atmospheric stability and dryness lead to the formation of the great arid belts, which include the Sahara Desert in North Africa, the Arabian Desert, the Australian Outback, and the Atacama Desert in South America. Their aridity is a direct consequence of the global atmospheric conveyor belt that removes moisture near the equator and delivers extremely dry, sinking air to the subtropics.